Canberra is on high alert following the terrorist attack in London, with armed police officers wielding semi-automatic weapons guarding Parliament House.

Heavily-armed officers were deployed early Thursday morning following the atrocity outside Britain's Westminster, with several guards seen outside federal buildings in the Australian capital.

The beefed up security was in place as Prime Minister Malcolm Turnbull vowed to 'defeat and destroy' terrorists seeking to kill Australians.

Mr Turnbull said the government would not allow terrorism or fear to affect the Australian way of life and said: 'We will never, ever let the terrorists win.'

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Armed police are pictured on patrol at Parliament House in Canberra, which is on high alert

Prime Minister Malcolm Turnbull vowed to 'defeat and destroy' terrorists seeking to kill Australians after four people died in the horrific London attack

Officers with guns were guarding Parliament on Thursday morning following the horrific events in London

Extra officers were drafted in overnight following the atrocity outside Britain's Parliament, with the government taking extra precautions

'We will defeat and destroy them on the battlefield, we will defeat and defy them at home,' the prime minister said.

'We will never change the way we live. We will never let them divide us. We will never let them intimidate us or challenge or democratic way of life or the freedoms for which generations of Australians have serve said and died to keep - served and died to keep secure.'

Mr Turnbull added that there would be extra security in Canberra in response to the attack in London.

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'The AFP commissioner has confirmed there will be an increase in police present in parliament house here in Canberra today,' he said.

'We are very alert to the vulnerabilities of places of mass gathering and the risk of lone attackers, like the perpetrators, the terrorist in London.'

Foreign Minister Julie Bishop earlier confirmed that no Australians had been caught up in the attack.

Canberra was placed on high alert this morning following the attack in Westminster on Wednesday afternoon, Mr Turnbull said

No Australians are believed to have been caught up in a horrific terror attack near London's Houses of Parliament that has left at least four people dead. Pictured, emergency services at the scene

Paramedics are seen treating the suspected attacker, who later died in hospital

Foreign Minister Julie Bishop said Australia's High Commissioner in London had not identified any Australians among the killed or injured following the atrocity. Pictured, emergency services at the scene

'I can assure Australians that to this point no Australians have been identified as being involved,' she said.

Ms Bishop said the Australian government was doing everything in its power to keep the country safe but warned that lone wolf terrorist attacks 'can occur anywhere at any time'.

She added that Australian extremists fighting for ISIS in Syria and Iraq may return as the barbaric group are defeated in the Middle East.

'They may well seek to come back to our part of the world. That's why we are monitoring them, tracking them, exchanging intelligence real-time, information to ensure we can keep people as safe as possible,' Ms Bishop told 9 News.

People ... may seek to come back to southeast Asia, Australia, we will do everything we can to track them and prosecute them if we have evidence to prove they are guilty of terrorism offences.'

Ms Bishop extended the Australian government's deepest sympathy to the people of the UK after speaking to her counterpart Boris Johnson.

The minister insisted the government is doing all it can to ensure Australia is safe from such an attack.

'But as we have seen in the United Kingdom today, these events can occur,' she said.

Mr Turnbull earlier tweeted, saying that Australia's heartfelt sympathies were with the victims of the attack.

'Australia stands in resolute solidarity with the people of Britain in war against terrorism,' he tweeted.

Opposition Leader Bill Shorten pledged support to Britain after the attack.

'This shocking crime, designed to weaken the UK, will only make her stronger. All Australians stand with Britain today,' he tweeted early on Thursday.

Foreign Minister Julie Bishop said no Australians were caught up in the attack in London

An attacker brought carnage to central London on Wednesday afternoon, mowing down pedestrians on Westminster Bridge and hacking at police with knives in the grounds of the Houses of Parliament

At least 20 people were hit when a 4x4 drove along the pavement on the crowded bridge, knocking down and seriously injuring pedestrians before crashing into a fence below Big Ben

There was carnage in central London on Wednesday afternoon when an attacked mowed down pedestrians on Westminster Bridge and hacked at police with knives in the grounds of the Houses of Parliament.

At least 20 people were hit when a 4x4 drove along the pavement on the crowded bridge, knocking down and seriously injuring pedestrians before crashing into a fence below Big Ben.

The killer, described by witnesses as 'middle-aged and Asian', then managed to break into the grounds of the Parliament where he fatally stabbed a police officer with two knives.

The policeman died at the scene. The attacker, who was shot by armed officers, died after he was taken to hospital.

It is believed one attacker was involved and he killed three people, including the policeman, and left at least 20 pedestrians and three other police officers seriously injured.

Prime Minister Theresa May was bundled into her car by a plain-clothes police officer and driven quickly from the scene as the attack unfolded.

London's Metropolitan Police said the attack, which comes a year to the day after the terrorist atrocities in Brussels, is being treated 'as a terrorist incident until we know otherwise'.

ABOUT TIME: SECURITY UPGRADE PARLIAMENT HAS LONG-NEEDED

A controversial security upgrade at Parliament House in Canberra is designed to ward off precisely the kind of attack seen on the UK parliament overnight.

Australian politicians last year approved a host of measures to lessen the building's vulnerability to a terror attack, including a 2.6 metre steel fence around much of the site's perimeter.

Critics of the controversial fence have likened it to putting barbed wire on the Opera House.

But the events in London overnight may silence those critics.

Australia's top security agencies have left parliamentarians in no doubt about the need to better protect the nation's seat of power.

The Australian Federal Police, ASIO, the Attorney-General's Department and other agencies have all supported the fence, which restricts public access to the sloping lawns at the front of Parliament House.

Other upgrades include the installation of more bollards to guard against ram attacks, and dozens of new security cameras.

Ironically, while Senators were giving the nod to the upgrade back in December, protesters staged a stunt that showed how vulnerable the building really was.

Pro-refugee campaigners managed to scale the building's facade to unfurl a banner, while others waded out into a water feature in parliament's forecourt and dyed the water blood red.